Effects of antifoam and scale-up on operation of bioreactors were experimentally investigated using two external loop airlift reactors. The volumes of the small and large reactors were 55 and 700 1, respectively. The addition of minute quantities of silicone-based antifoam lead to reduction in both hydrodynamic and mass transfer characteristics of all reactors. New simple empirical correlations were found for the reactors design parameters: gas hold-up, liquid circulation velocity and volumetric mass transfer coefficient. No discernible relation of antifoam concentration with scale-up of airlift reactors was found for all the design parameters.